1. Field of the Invention
This invention relates to shielded flat cables in the form of a flexible strip including conductors and films attached to the conductors in the upper and lower surfaces thereof. Such flat cables are used to perform electrical wiring for various equipment or instruments in order to prevent leakage of electromagnetic waves exteriorly from the conductors inside the flat cable, or to prevent adverse effect of electromagnetic waves.
2. Statements of Prior Art
Numeral types of shielded flat cables are known in the art. For example, Japanese Patent Public Disclosure No. 4-33211, and Japanese Utility Model Publication No. 4-51712 disclose grounding constructions for circuit conductors and ground conductors juxtaposed therewith. Such grounding constructions may be classified into two types as summarized below.
For convenience of illustration, shielded flat cables in prior art will be explained below with reference to FIGS. 7 through 10.
FIG. 7 is an exploded perspective view of a shielded flat cable in prior art. FIG. 8 is a cross-sectional view along line 8--8 of FIG. 7. FIG. 9 is an exploded perspective view of an another shielded flat cable in prior art. FIG. 10 is a cross-sectional view along line 10--10 of FIG. 9.
One example of the flat cables in the prior art includes, as shown in FIGS. 7 and 8, circuit conductors 1 and ground conductors 2 which are alternately juxtaposed on a base film 3. A cover layer film 4 is adhesively placed over the conductors 1, 2, so that the conductors are sandwiched in between the base film 3 and the cover layer film 4. Several portions of the cover layer film 4 are cut out to form small connection windows 6 through which the ground conductors 2 are exteriorly exposed. A pair of shield tapes 5 of a conductive metal foil are attached to the outer surfaces of the cover layer film 4 and base film, respectively, so as to shield the cable. The portions of the ground conductors 2 exposed through the connection windows are conductively connected to the shield tape 5 by means of junction, deposition, welding or the like for the purpose of grounding, while, on the other hand, the circuit conductors 1 are electrically insulated from the shield tape 5 by means of the cover layer film 4.
The other type of shielded flat cable includes, as shown in FIGS. 9 and 10, a plurality of circuit conductors 1 juxtaposed on a base film 3, and a ground conductor 2 disposed on one side of the circuit conductors in parallel relationship with the circuit conductors. A cover layer film 4 is adhesively attached to the base film 3 so that it covers the circuit conductors 1, but not the ground conductor 2. A pair of shield tapes 5 are adhesively connected to the cover layer film 4 and base film 3 for the purpose of shielding. The ground conductor 2, not covered with the cover layer film 4, is conductively connected and grounded to the shield tape 5.
With the first mentioned flat cable, it is difficult and time consuming to form small connection windows 6 in the intermediate portion of the cover layer film 4. Formation of such connection windows becomes Impossible in substance when it is intended to make the spaces between the conductors narrower. By this, spaces between the conductors are limited to an amount above a predetermined value. Thus, this type of flat cable is disadvantageous, since it does not meet the requirement in the art in view of miniaturization and cost savings of the flat cables. It is also noted that a space does exist between the ground conductors 2 in the area of the connection windows and the shield tape 5, the space corresponding to the thickness of the cover layer film 4. Thus, it is difficult to perform soldering work for electrically connecting the ground conductors 2 to the shield tape 5. A large amount of soldering material is required. Reliability in electrical connection between the ground conductors and the shield tape is not sufficient, since such electrical connection is made by means of soldering material filled in the above space.
With the secondly mentioned shielded flat cable in prior art, the ground conductor, 2 exposed exteriorly along the length thereof, may be directly connected to the shield tape 5. Thus, the "less workability", "limitation in the space between the conductors", and "less reliability in electrical connection", due to the presence of the connection windows 6, mentioned above may be eliminated. It is noted, however, that sufficient "shield effect" cannot be obtained since the ground conductor 2 is distant from the circuit conductors 1.